Children with aganglionic colon present with a constellation of symptoms (e.g., severe constipation, diarrhea, intestinal inflammation) that predispose to sepsis, and can be life-threatening. Surgical correction eradicates these symptoms in most cases. But in up to 30% of cases, surgery fails to cure the disorder, and thereafter, children suffer from chronic enterocolitis that can lead to bacteremia, outright sepsis, and even death. Almost nothing is known about the underlying mechanisms of this postoperative enterocolitis. As a means to pursue mechanistic studies, we created an animal model of postoperative enterocolitis associated with aganglionosis using the Endothelin Receptor B-null mouse with colonic aganglionosis. We developed a novel microsurgical procedure that is essentially identical to the operation performed clinically, and essentially cures aganglionic megacolon in Ednrb-/- mice, just as it does in human patients. We found that a significant proportion of Ednrb- /- mice develop postoperative enterocolitis despite satisfactory surgical outcome, similar to what occurs clinically in human patients. In the course of our studies, we serendipitously discovered a spectrum of striking immune abnormalities in Ednrb-/- mice. These abnormalities occur at or shortly after birth, appear to worsen with age, and include reduced spleen size, reduced absolute numbers of lymphocytes in the spleen, marked reduction in marginal zone B cells and B cell/T cell ratio in the spleen, thymic involution, and bone marrow abnormalities. Upon examination of mice with a genetic lack of entdothelin-3 (ET3; the preferred ligand for the Ednrb receptor), we found very similar immune abnormalities. Our preliminary studies thus far are consistent with the interpretation that ET3 signaling through Ednrb is required for normal development of the immune system, and/or for normal function of the immune system in the adult animal. We therefore propose that 1) ET3/Ednrb signaling is essential for normal development of the immune system; 2) when disrupted, two distinct phenotypic features emerge: colonic aganglionosis, with a marked propensity to develop enterocolities even after successful surgical correction, and an immune system that displays marked structural and functional abnormalities. We suspect that these two phenotypic features are indeed related to one another, and our ultimate goal is to understand and describe in molecular and cellular terms the precise basis of the interrelationships. This K08 application proposes an a detailed, comprehensive training plan that will allow the applicant to pursue immunologic studies related to disrupted ET3/Ednrb signaling. An outstanding team of Mentors will directly oversee the research training and the all aspects of the investigation. The ultimate goal is for the applicant to leverage the data obtained during this proposed investigation into a successful R01 application. This will then allow the applicant to become an independently-funded surgeon-scientist investigating the role of specific signaling pathways and genes in immune development and function, and how these are linked to colonic ganglionosis and the associated enterocolitis that occurs in these diseases.